Anyone question is ok, but it pays to do a little research before asking or atleast ask a detailed one. Go to google and type in chicken-egg startup. There are hundreds of articles and discussions on that issue.
In your case, it doesn't look like you have your biz-idea polished since you are asking for ideas on what users will be doing on your platform.
Here's my 2 cents, pick a small area, I'm assuming you know a bit on the solutions to the problems other users might need, so have a network of friends or people you know who can atleast satisfy some of the needs signed up and start participating. Disregard the time it will take to meet a need since your goal is to make sure a need is solved (satisfied need = word of mouth spread). From there, start reaching out to other end users (needs-satisfiers) and start recruiting them, vice versa.
Hint: before Uber drivers network grew, the founders were actually giving rides themselves.